Lead frames are made of thin metallic plates in which patterns such as inner leads and outer leads are formed with predetermined gaps left between the leads. In such a lead frame, after mounting a semiconductor chip thereon, terminals of the semiconductor chip and the inner leads are electrically connected to each other.
Recently, there has been an ever stronger demand for a higher packing density and higher integration of semiconductor elements. Corresponding to the demand, lead frames for mounting semiconductor elements thereon have also been developed which are of fine shapes and highly accurate dimensions. In QFP type semiconductor devices, particularly, great weight has been given to increase the number of pins per lead frame. At the present, a lead frame having the number of leads in excess of 300 pins is manufactured. With an increase in the number of pins per lead frame, it has been required to develop a lead frame wherein inner leads adjacent to a semiconductor element are smaller in pitch and finer in shape. For example, while the inner lead pitch has been about 200 .mu.m at minimum in the prior art, the pitch smaller than that level is required nowadays.
Such a lead frame having a number of pins and a small pitch has been primarily processed by etching and pressing in the past. By these processing methods, the lead frame can be manufactured relatively efficiently and easily.
JP, A, 2-247089 discloses a method of forming the portions where the pitch is not so small, such as outer leads and notches around a die pad, by the pressing or etching process, and then forming inner leads by laser cutting successively. In this method, only the inner leads having a small pitch is processed by laser cutting.
JP, A, 4-37493 discloses a method by which the portions which have been etched but not thoroughly the total thickness of a plate to be processed, i.e., the portions where the plate has not been pierced in the direction of plate thickness by etching, are subject to irradiation of a laser beam in a post-process so as to pierce the plate.
JP, A, 62-93095 discloses a method of changing (expanding or contracting) the section of a laser beam into an elongate elliptical shape by an optical system including a cylindrical lens. With this method, in the case of processing with fine dimensions, a long gap can be formed by a single irradiating step of the elongated laser beam (hereinafter referred to as elliptical laser beam).
Though not concerned with laser cutting, JP, A, 59-72754 discloses a method of leaving joint portions interconnecting inner leads at near their distal ends when a lead frame is fabricated by pressing, then removing residual strains generated during the pressing by heat treatment, and finally cutting the remained joint portions. The publication of this prior art describes that, by leaving a joint portion interconnecting inner leads at near their distal ends, release of processing strains generated during the pressing is suppressed to prevent deformations of the leads and lead positions are not shifted even with heat treatment or the like effected to remove the processing strains.